Efficient temperature forcing of semiconductor devices under test

ABSTRACT

A temperature-forcing system and method for controlling the temperature of an electronic device under test comprises a temperature-forcing head, including a face positionable in thermal contact with the device, and an evaporator, in direct or indirect thermal contact with the face; and a refrigerant circulation subsystem, including a compressor, a condenser, a flow control device for inducing a pressure drop in the refrigerant, and a conduit circuit through which the refrigerant is flowable. The subsystem cooperates with the evaporator so as to define at least one closed loop through which a corresponding bi-phase refrigerant is circulatable, so that, during circulation, the refrigerant is maintained in a liquid phase between the compressor and the flow control device and in a gaseous phase while flowing through the evaporator. The temperature of the device is therefore switchable by the head at a rapid rate of 50 to 150 degrees Celsius per minute.

The present application is a continuation-in-part of U.S. applicationSer. No. 13/405,870 filed Feb. 27, 2012 and bearing the same title.

FIELD OF THE INVENTION

The present invention relates to temperature forcing of electroniccomponents and, in particular, to temperature forcing of semiconductorchips and modules during testing.

BACKGROUND OF THE INVENTION

In the electronic components semiconductor industry it is generallyrequired to subject prototypes and production samples of semiconductordevices (chips or modules) to thorough electrical testing. Sincespecifications of a device typically include the range of ambienttemperatures over which it should be operable, each device under test(DUT), and more specifically its casing, must generally be held duringpart of such testing at each of the extreme temperature values of thespecified range (thus simulating the required extreme ambienttemperature values). Such extreme values are typically between 125 and165 degrees centigrade, at the high end, and between −40 and −70 degreescentigrade, at the low end. The process of thus keeping the casetemperature of a DUT at one or the other of the specified extreme valuesis known as temperature forcing and is achieved, in common practice, byplacing a heat conducting device in tight thermal contact with the DUT'scasing and controlling its temperature so as to be held near the desiredvalue. The heat conducting device and the system that controls itstemperature are together referred to as a temperature forcing system(TFS).

Moreover, as is well known, operation of a semiconductor chip or anyother electronic component is an exothermic process, wherein electricpower fed to it is converted to heat, thereby tending to raise thetemperature of the chip. This heat must be dissipated by the ambient airand/or external devices, as well as, generally, by the temperatureforcing system, in order to limit the rise of the temperature, leavingthe latter in equilibrium at the desired level. This is particularlytrue when testing at the low temperature range. When testing at a highextreme temperature, however, the exothermic process of the DUT may attimes be insufficient to raise its casing temperature to the desiredlevel, as all of the generated heat is dissipated by the ambient air andexternal devices. In this case the temperature forcing system, ratherthan dissipating heat, must supply heat to the DUT through its casing.

Two important requirements govern such temperature forcing: Onerequirement is that the temperature of the DUT, or its casing, bemonitored and held at the desired level quite accurately and constantly(say—within 0.1 degrees C.). The other requirement is that thecontrolled temperature be switchable between the two extreme values (orto any other values) within a relatively short time (say—at a rate of10-60 degrees C. per minute). It is noted that the temperature must beheld constant even while varying operations in the chip (per testprocedures), causing varying amounts of heat to be generated therein;the present invention aims at holding the DUT's temperature constanteven while the input power dissipated by the DUT varies between afraction of a watt and several hundred watts. Another, obvious,requirement is that any test setup be operational with a wide variety ofchip types to be tested, having different heat-generatingcharacteristics.

During testing, a semiconductor device (e.g. a packaged chip or anelectronic module) is typically held in a test jig so that electricalterminals on its bottom surface are in contact with appropriatelyconfigured electrical test circuitry, while its top surface isaccessible for temperature forcing. Other test configurations are alsopossible and are equally addressed by the present invention.

A typical temperature forcing system of prior art comprises a thermalhead that is placeable in thermal contact with the DUT, a chiller and acirculation system that circulates a heat transfer fluid between twoheat exchangers—one in the chiller and one in the thermal head. Thechiller is a conventional refrigeration system, operational to extractheat from its heat exchanger and thus to cool the heat transfer fluid tosubstantially below the extreme low temperature of the desired testingrange. The heat transfer fluid is generally designed to remain liquidthroughout the circulation system and over the entire range of thetesting temperatures. When passing through the thermal head's heatexchanger, the transfer fluid extracts heat from the thermal head, thus,in turn, cooling it.

Such a prior-art system has three major drawbacks: (a) The presence oftwo heat exchangers in tandem causes a relatively large cumulativetemperature differential between the chiller and the DUT, thus reducingthe efficiency of the process and placing a sometimes unacceptable limiton how low the temperature of the latter may be forced with a simple(single-stage) refrigeration system. (b) Heat dissipation in the thermalhead's heat exchanger is based on the principle of Forced Heatconvection, whose heat transmission factors are low (by several ordersof magnitude relative to the principle on which the present invention isbased); this seriously limits the rate at which the DUT temperature maybe changed during testing. (c) The relatively large heat capacity of thecirculating transfer fluid further limits the rate at which thetemperature may be switched.

It is an object of the present invention to provide atemperature-forcing system for controlling the temperature of anelectronic device under test more quickly and accurately than one of theprior art.

Other objects and advantages of the invention will become apparent asthe description proceeds.

SUMMARY OF THE INVENTION

The present invention improves the process of temperature forcing of anelectronic device under test, over prior art, by providing that thecooling of the device, or of a member in direct or indirect thermalcontact therewith, be effected directly by the evaporation of a bi-phaserefrigerant, rather than by the flow of an intermediate single-phasetransfer fluid (gaseous or liquid) that, in turn, exchanges heat with aremote cooling apparatus. One important advantage of this arrangement,over prior art, is that it involves fewer stages of heat transfer, withtheir associated temperature differentials, thus achieving greaterthermal efficiency—enabling, for example, to achieve lower devicetemperatures for a given set of refrigeration parameters or, conversely,to utilize a smaller and less powerful, compressor to attain a givendevice temperature. Another important advantage of this arrangement, tobe explained below, is the greater speed at which the device temperaturemay be switched.

Basically, a system according to the present invention comprises acentral unit, at least one temperature-forcing head (termed in thesequel interchangeably also “thermal heads” or “heads”, for short),attachable to device-holding test jigs, each connected by a pair ofsupply- and return tubes to the central unit, and a bi-phase refrigerantthat circulates throughout this combination. During operation, for eachactive temperature-forcing head, the refrigerant flows, in generallyliquid state, from the central unit, through the supply tube to thehead, where some or all of it evaporates while dissipating heattherefrom and whence it returns, generally in a mixed liquid- andgaseous phase, through the return tube, to the central unit and then itrecirculates.

The central unit basically includes a compressor and a condenser (whichis preferably coupled with an atmospheric heat exchanger). The headbasically includes a thermal contactor, having a face configured to beput in thermal contact with the electronic device, and an evaporatorpart, directly or indirectly in thermal contact with the thermalcontactor, and particularly with the heat spreader. In someconfigurations the thermal head, and particularly the thermal contactorpart, includes a thermoelectric cooler (TEC), in direct thermal contactwith other portions of the head. The term “thermal contact” (between twocomponents) is here used to denote either (in the case of direct thermalcontact) a direct physical contact between the components that allowsheat transfer between them or (in the case of indirect thermal contact)a physical contact between each of the components and an intermediary,mutually adjacent, heat conducting member.

The evaporator is formed as a flow-through enclosure and includes aninlet port and an outlet port, connected with the supply tube and thereturn tube, respectively, thus being in the flow path of therefrigerant. Additionally in the flow path, between the condenser andthe evaporator, there is a metering device, such as (but not limited to)an expansion valve or a capillary, which is a flow-restrictingcomponent, configured to continuously, adjustably or selectively createa pressure differential between the high pressure of the inflowingrefrigerant produced by the compressor and the resulting low pressure inthe outflowing refrigerant exiting the metering device.—and then causedto flow through the evaporator. The term high pressure is defined hereas that pressure at which the refrigerant assumes the liquid phase whenat normal atmospheric temperature and the term low pressure—that atwhich the refrigerant assumes the gaseous phase even when at the lowesttemperature at which the evaporator is to operate. Part of theevaporator is preferably formed as a heat exchanger, configured toefficiently exchange heat between the refrigerant and the other parts ofthe head, so as to generally dissipate heat therefrom, (and thereby—fromthe DUT).

Preferably the end portion of the thermal contactor that comes inthermal contact with the device under test, known as heat spreader, isformed to have an end face that matches the upper face of the device insize and shape. Preferably the thermal contactor is configured as anexchangeable component of the temperature-forcing head, there beinggenerally a plurality of thermal contactors adapted to be part of anyone head. The plurality of thermal contactors may differ mutually in theshape of the heat spreader, so as to be usable with a correspondingplurality of device types. If the head is configured to include a TEC,the latter is preferably included in the thermal contactor. Thus theplurality of thermal contactors may also differ mutually as to whetheror not they include a TEC and if they do—may differ in the type of TECincluded.

Preferably there is at least one heat sensor imbedded in each of theevaporator wall and the heat spreader (in the thermal contactor),configured to provide temperature feedback signals, as described below.Also embedded in the heat spreader, in some configurations, is one ormore heating elements, as described below.

Operation of the system is generally as follows: Firstly, therefrigerant which has been pressurized by the compressor is convertedfrom the gas to liquid phase in the condenser, through heat dissipationby an atmospheric heat exchanger, or any other type of heat exchanger.When a low temperature is desired at the device under test, therefrigerant is made to flow through the metering device. The liquidrefrigerant emerging from the metering device enters the evaporator,where low pressure prevails, wherein it comes into contact with the heatexchanger, or possibly with any other part or wall of the evaporator, toevaporate thereon into gaseous phase and thereby to dissipate heattherefrom and thus also—through the thermal contactor (and possiblythrough a TEC therein)—from the DUT. The resulting low pressure mixtureof gas and liquid is drawn by the compressor, through the outlet portand the return tube, for another cycle. It is noted that the system thusoperates essentially as a conventional cooling system, but wherein theevaporator and its associated heat exchanger are advantageously part ofthe temperature-forcing head, serving to dissipate heat therefrom in ahighly efficient, as well as effective, manner. The efficiency isinherent to the topical evaporation (i.e. boiling) process, with itsextremely high heat transfer coefficient, as well as to the singleheat-exchange process (in contrast to double heat-exchange processesthat typify prior-art systems), while effectiveness characterizes thefact that the heat-exchanger and the parts of the head thermally coupledthereto nearly reach the very low boiling temperature of therefrigerant.

In order to coarsely adjust the temperature at the evaporator (andthereby—the temperature forced on the DUT), the rate of heat dissipationtherefrom may be alterable, by changing the rate—preferably the averagerate—at which evaporating liquid refrigerant flows through theevaporator; this can be achieved by any of several means, depending onsystem configuration. Such adjustment is preferably controlled by usingtemperature feedback signals from one or more heat sensors imbedded in awall of the evaporator. Preferably the control operation is by means ofa central control unit. Such control capability is also applicable tocoarsely maintaining a given temperature value at the evaporator (andthereby—at the face of the heat spreader) even while heat dissipationrequirements vary under varying operations at the DUT (e.g. as dictatedby test procedures).

When a high temperature is desired at the DUT (such as an extreme hightemperature dictated by the test procedures), the operation describedabove can be modified so as to drastically reduce, or totally eliminate,heat dissipation from the evaporator. One manner of such modification isto drastically reduce the rate at which refrigerant flows through theevaporator—essentially as described above; in an extreme situation, theflow may be stopped altogether. Another possible modification is tocause the refrigerant to flow through the evaporator in gaseous phaseonly; in this case, generally no condensing and no evaporation takesplace. If it is necessary to supply heat to the DUT (rather than todissipate heat therefrom) in order to maintain it at a high temperature,heating elements in the heat spreader may be activated; if a TEC isincluded in the head, it may be run with reverse current, to supply heatas required.

Switching between high and low temperature levels, as is often requiredduring testing, is effected by selectively applying the chosenmodification to the flow of the refrigerant. It is noted that each ofthe two diverse temperature levels is mainly effected by a correspondingstate of refrigerant flow through the evaporator; that is—a lowtemperature level is effected by a high average rate of flow of therefrigerant, in a state of evaporation, with consequent high rate ofcooling of the heat exchanger, whereas a high temperature level iseffected by a relatively low, or even zero, average rate of flow in thestate of evaporating liquid or by the flow in gaseous phase only, withconsequent low or zero rate of cooling. Since the change in the rate orin the phase mode occurs within the head and since the switching of theaverage rate or of the mode can be practically instantaneous, theresultant temperature change in the heat exchanger—and hence also in thedevice under test, which is in thermal contact therewith—is extremelyfast. This is an important advantage of the invented system andcontrasts with systems of prior art, wherein the temperature of anintermediary coolant must be switched, which takes a much longer time,owing to the relatively high heat capacity of the voluminous coolant andowing to the two (rather than one) heat exchange processes involved.

A high rate of switching of at least 50° C./min between high and lowtemperature levels is made possible by employing the aforementionedrefrigeration cycle within the temperature-forcing head to benefit fromat least the following factors: (1) direct cooling of the DUT, (2) arelatively low level of thermal inertia due to selection of a suitablerefrigerant that is bi-phase at relatively low pressures, and (3) alarge convective heat transfer coefficient on the order of 20,000 W/(m²°K) resulting from refrigerant boiling and the release of latent heat ofvaporization during a liquid to gas phase change. The temperatureswitching rate will generally not exceed 150° C./min due to materialdeformation considerations.

As mentioned above, in some configurations of the invented system thereis interposed between the heat exchanger and the heat spreader(preferably as part of the thermal contactor) a thermoelectric cooler(TEC), in thermal contact with both of these components. Its purpose isprimarily to enable highly accurate and very fast control of thetemperature in the DUT. Such control is achieved by sensing thetemperature of the DUT or of the heat spreader—preferably by means of asuitably embedded sensor—and accordingly controlling the averageelectric current flowing through the TEC. At times, the TEC may alsoserve to provide additional temperature shift, to achieve temperatureranges at the DUT beyond what is achievable by the cooling subsystemalone. Thus, for an extremely low temperature at the DUT, the TEC isoperated in its normal, cooling mode with a relatively high value ofelectric current; its face next to the heat spreader is thus kept at anappreciably lower temperature than that of the heat exchanger (as iseffected by the coolant evaporation). The converse is true when aimingat an extremely high temperature at the DUT; if the heat produced by thelatter is insufficient to raise its casing temperature to the desiredlevel, even at zero dissipation by the cooling subsystem (i.e. completestoppage of coolant circulation), the TEC may be operated in a reversemode (i.e. with a reversed current flow), so that its face next to theheat spreader is at a substantially, yet controllably, highertemperature than the ambient temperature, causing an additional,temperature rise at the interface to the DUT and enabling the latter'stemperature to be kept at the desired elevated level. To raise theforcing temperature even further, the current through the TEC may beincreased to a level at which resistive (ohmic) heating takes effect.

In some configurations of the invented system, electrically resistiveheating elements may be imbedded in one component or another of thethermal head (such as the heat exchanger or the heat spreader), in orderto introduce additional heat and thus raise the temperature at the DUTto a desired high level. In configurations lacking a TEC, such a heatingelement may also be utilized to accurately control the forcedtemperature, by varying the magnitude of the electric current flowingtherethrough (again, under control of a feedback signal obtained from atemperature sensor imbedded in the heat spreader or in the DUT itself);this mode of temperature control may also be applied for the case of lowtemperatures, by thus adding a small but controllable amount of heat tothat dissipated by the highly cooled heat exchanger.

There is thus disclosed herein a temperature-forcing system, forcontrolling the temperature of an electronic device under test,comprising a temperature-forcing head, including a face positionable inthermal contact with said device, and an evaporator, in direct orindirect thermal contact with said face; and a refrigerant circulationsubsystem, including a compressor, a condenser, a flow control devicefor inducing a pressure drop in said refrigerant, and a conduit circuitthrough which said refrigerant is flowable.

Said subsystem is configured to cooperate with said evaporator so as todefine at least one closed loop through which a corresponding bi-phaserefrigerant is circulatable, so that, during circulation, saidrefrigerant is maintained in a liquid phase between the compressor andthe flow control device and in a gaseous phase while flowing through theevaporator, wherein the temperature of said device is switchable bymeans of said head at a rate of 50 to 150 degrees Celsius per minute.

More specifically, in the disclosed system the refrigerant, whileflowing through the evaporator at a low pressure, is operative todissipate heat therefrom by evaporation and the evaporator or any partthereof is formed as a heat exchanger.

In various configurations of the system, the head further includes athermo-electric cooler, one or more resistive heaters or one or moretemperature sensors.

In some configurations of the system, the head is formed as two partsthat are mutually attachable and detachable, one part being a thermalcontactor, which includes the face that is configured to be put inthermal contact with a device under test. The thermal contactor mayfurther include a thermo-electric cooler.

In one embodiment, the circulation subsystem is a two stagerefrigeration cycle by which a first stage refrigerant pressurized by afirst compressor is delivered to a heat exchanger whereat it cools asecond stage refrigerant pressurized by a second compressor to apressure greater than the pressure generated by said first compressor,for an increased cooling effect.

As additional features of the disclosed system, the refrigerantcirculation subsystem further includes a bypass conduit, configured toselectively provide a flow path for the refrigerant that avoidscirculation through the evaporator, the subsystem is operative tocirculate the refrigerant intermittently, with a variable duty cycle, orat a variable rate of flow and the metering device is an adjustableexpansion valve.

In some configurations, the system further comprises a tube assembly,attached to the head and configured to pass the refrigerant from thesubsystem to the evaporator and back from the evaporator to thesubsystem, wherein the tube assembly is connectable to, and detachablefrom, the subsystem. In some configurations, the system comprises one ormore additional temperature-forcing heads, each similar to the disclosedtemperature forcing head, wherein the subsystem is configured tosimilarly circulate the bi-phase refrigerant also through the evaporatorof each of the additional heads.

Also disclosed herein is a temperature-forcing head, for controlling thetemperature of an electronic device under test, comprising a thermalcontactor, configured with a face adapted for thermal contact with thedevice and an evaporator, in direct or indirect thermal contact with thethermal contactor and configured to be connectable to a circulationsystem for circulating a bi-phase refrigerant in a gaseous phase throughthe evaporator and in a liquid phase through a condenser, wherein thetemperature of said device is switchable by means of said head at a rateof 50 to 150 degrees Celsius per minute.

More specifically, in the disclosed head the evaporator or any partthereof is formed as a heat exchanger and is configured to transfer heatto any bi-phase refrigerant, entering it in liquid phase and flowingtherethrough at a low pressure, by evaporating the refrigerant.

Also disclosed herein is a method for dissipating heat from a thermalhead while in thermal contact with an electronic device under test,comprising (i) providing a bi-phase refrigerant and a circulation systemtherefor, in fluid communication with a portion of the thermal head; and(ii) causing at least some of said refrigerant, while in liquid phase,to come in thermal contact with said portion, whereby at least some ofthe refrigerant evaporates, wherein the temperature of said device isswitchable by means of said head at a rate of 50 to 150 degrees Celsiusper minute.

The present invention is also directed to a method for forcing thetemperature of an electronic device under test, comprising (i) providinga thermal head, including an evaporator part, and placing it in thermalcontact with the device; (ii) connecting the head to a circulationsystem and causing said system to circulate a bi-phase refrigerantthrough the evaporator part; and (iii) when heat dissipation from thedevice is required, causing at least part of said refrigerant to enterthe evaporator in liquid phase, whereby it dissipates heat therefrom byevaporation, wherein the temperature of said device is switchable bymeans of said head at a rate of 50 to 150 degrees Celsius per minute.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic overall block diagram of a temperature-forcingsystem according to one embodiment of the present invention;

FIG. 2A is a schematic isometric drawing of an embodiment of thetemperature forcing system of FIG. 1, showing a central unit and athermal head;

FIG. 2B is a schematic isometric drawing of an alternative configurationof the embodiment of FIG. 2A;

FIG. 3 is an axial-sectional view of an embodiment of the thermal headin the system of FIGS. 2A and 2B, in one configuration of the system;

FIG. 4 is an isometric drawing, in top open view, of an embodiment ofthe evaporator and heat exchanger in the thermal head of the system ofFIGS. 2A and 2B;

FIG. 5 illustrates in external view an alternative configuration of thethermal head of FIG. 3; and

FIG. 6 is a schematic illustration of a thermodynamic cycle operating inconjunction with another embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is made to both FIG. 1, which shows a temperature-forcingsystem according to an embodiment of the invention schematically inblock diagram manner, and FIG. 2A, which depicts a particular embodimentof the system in an isometric view. The system is generally configuredand operative to circulate a bi-phase refrigerant through atemperature-forcing head (thermal head).

A central unit 10, usually with an enclosing case (not shown), has acompressor 12, a condenser 13 in intimate thermal contact with anatmospheric heat exchanger, and an expansion valve 18. A pipe (notshown) connects the outlet of compressor 12 with the inlet of condenser13, and another pipe 17 connects the outlet of condenser 13 to the inletof expansion valve 18. The heat exchanger is in thermal communicationwith the atmosphere, aided by a fan 14. Also within the central unit 10is a controller 15, in electrical communication with a control panel 16.

A tube assembly 20, preferably flexible, connects central unit 10 with athermal head 30. In the presently illustrated configuration there is asingle thermal head, but in other configurations there may be two ormore thermal heads with their respective tube assemblies, connected inparallel to the central unit. The tube assembly includes a pair oftubes—a supply tube 21 and a return tube 22—as well as an electric cable23 (which includes a number of wires). The inlet end of supply tube 21is connected to the outlet of expansion valve 18, while the outlet endof return tube 22 is connected to the inlet of compressor 12. Theelectric cable 23 is connected to controller 15. It is noted that, withrespect to the thermal head, the central unit 10 and the tubes 21 and 22jointly form a refrigerant circulation system.

Expansion valve 18 is a preferred type of what may be generally referredto as a metering device, which is a fluid flow regulating component withan essentially narrow passageway that is configured to restrict flow ina manner that, in cooperation with the compressor, creates a pressuredifferential across it. In other words, it is operative to allow highpressure to be built up upstream to it (by the action of compressor 12),whereby the refrigerant is kept in liquid phase, while allowing lowpressure to be maintained downstream to it, whereby the refrigerant isallowed to evaporate. The metering device may also be any of severalother types, including, for example, a capillary tube. In some otherconfigurations or embodiments, it may be disposed as part of the supplytube or as part of the thermal head. Preferably and as in theillustrated embodiment, expansion valve 18 is adjustable, that is thedegree of stricture is variable; the narrower the passageway, the lowerthe rate of flow and the higher the pressure differential (up to themaximum achievable with a given compressor) and conversely—the wider thepassageway, the higher the rate of flow (up to the maximum achievablewith a given compressor) and the lower the pressure differential. Asexplained below, this adjustability may affect commensurate variabilityin the rate of heat dissipation from the thermal head and in the minimumtemperature achievable therein.

In the illustrated configuration there is within the central unit 10also a bypass conduit 19, leading from another outlet of the expansionvalve 18 to the inlet of compressor 12. Passage of refrigerant throughthe bypass conduit is switchable—preferably within the expansion valve.As explained below, it may serve, when required, to eliminate anypressure differential and thus to prevent any cooling effect.

In an alternative configuration of the system, shown schematically inFIG. 2B, the tube assembly 20 consists of two sections that areinterconnected by a set of matching connector pairs—connectors 24A forthe supply tube, connectors 24C for the return tube, and connectors 24Bfor the electric cable. A first section of the assembly is configured aspart of the central unit 10, the connectors at its end being preferablyaffixed to its case (not shown), while the second section is long enoughto reach the test setup. The connectors are preferably configured forquick connection and quick release, as is commercially available. Asuitable valve mechanism, commercially available, at each of the tubeconnectors blocks the refrigerant from leaking out while the tubes aredisconnected. This configuration enables easy removal of the head fromthe central unit for servicing or for replacement; it also enables thealternate use of several different heads—possibly with tube assembliesof different lengths.

Thermal head 30 is configured to make thermal contact with a deviceunder test, disposed in a test jig, and to thus dissipate heat from thedevice and (for the case of high-temperature testing) possibly supplyheat thereto. Thermal head 30 is connected to the other end of tubeassembly 20—in a manner further explained below—whereby, in particular,it is in fluid communication with the circulation system, providingfluid passage from supply tube 21 to return tube 22.

A bi-phase refrigerant, of any suitable type with low boiling point,such as Freon, including Freon R22, R134, R134a, R408A, R507 and R717,nitrogen, and carbon dioxide, but preferably Freon R23 (having a boilingpoint of −115.7° F. at 1 atm) and/or R404A (having a boiling point of−40.8° C. at 1 atm), closely circulates through the entire system, thatis—it generally flows (in the order listed) from compressor 12, throughcondenser 13, expansion valve 18, supply tube 21, thermal head 30, andreturn tube 22, back to the compressor.

By virtue of its low boiling point, the refrigerant undergoes cavitationin a turbulent flow regime while circulating through its conduitcircuit, providing a large convective heat transfer coefficient on theorder of 20,000 W/(m²*° K). It is thus an advantageous feature of theinvention that the refrigerant flows through the tube assembly and thethermal head—in contrast to a chiller non-boiling type system of theprior art which provides a convective heat transfer coefficient of only2000-5000 W/(m²*° K), where the refrigerant is confined to a centralunit, while a secondary coolant flows through tubes and the thermalhead.

In some configurations, wherein the tube assembly comprises twointerconnectable sections as described above, there can be provided aplurality of thermal heads 30, of different types and/or sizes, to servefor testing various types and sizes of devices, under various testingconditions. Each head is connected to a corresponding second section oftube assembly 20, interchangeably connectable to the central unit 10. Inother configurations, each head is structured to have a detachablecomponent thereof (the thermal contactor—to be described below), whichis to thermally contact the DUT, and there can be provided a pluralityof such components, interchangeably attachable to a thermal head andbeing of different types and/or sizes, to serve for testing varioustypes and sizes of devices, under various testing conditions.

FIG. 3 depicts schematically, in a cut-open isometric view, anembodiment of one configuration of a temperature-forcing (thermal) head30. As seen in FIG. 3 and FIG. 1 collectively, the head is fixedlyconnected to the tube assembly 20 and includes a housing 31 and anassembly of components that are sequentially in thermal contact witheach other. These typically consist of (in the listed order)—

-   -   a heat spreader 32,    -   a thermo-electric cooler (TEC) 33 and    -   an evaporator 40.

Mutual thermal contact between these components (represented in FIG. 1by wide double arrows) is achieved by flatness of their respectivecontacting surfaces and preferably enhanced by interposing a thin layerof heat-conducting substance, such as a thermal pad, thermal grease orIndium-based foil.

The housing 31 is adapted to mechanically engage test jig 102 so as tohold the thermal head in proper position with respect to thedevice-under-test (DUT) 100. The heat spreader 32, which is intended toensure that DUT 100 will be maintained at a substantially constanttemperature by which a maximum difference between an instantaneous highDUT temperature and low DUT temperature is no more than 0.10° C., has aface 42 configured to conform to the shape of, and to be placed inthermal contact with, the DUT 100. The heat spreader face 42 ispreferably in physical contact with the DUT 100 in order to induce heattransfer by conduction. Imbedded in the heat spreader is a temperaturesensor 39, available commercially, which is connected through wires 38in cable 23 to controller 15 and operative to sense the temperature ofthe heat spreader, and thus indirectly also of the underlying device,and to send a corresponding signal to controller 15 (FIG. 2A). In someconfigurations of the thermal head, the heat spreader also includes oneor more electrically resistive heating elements (not shown), to servefor supplying heat to the DUT when necessary; electric current issupplied to the heating elements from controller 15 through wires (notshown) in cable 23.

TEC 33 is a flat device, based on the Peltier effect, containing one ormore bi-metal couplers (in series), which are electrically connectedbetween two poles, to which direct voltage is applied during operation;the voltage is obtained through a pair of wires in cable 23 (which ispart of tube assembly 20) from controller 15 (FIG. 2A). TEC 33 may beany of a number of sizes and types available. The polarity and magnitudeof the applied voltage affects the nominal temperature differentialbetween the two faces of the TEC, e.g. the upper and lower faces. Theactual temperature differential is generally lower and depends on therate at which heat must be dissipated from the DUT and on the type ofthe TEC; in extreme cases the temperature differential may becomeinsignificant. Moreover, applying voltage of higher magnitude than thatrequired for maintaining the temperature differential may causesignificant current to flow through the TEC, resulting in ohmic losses,which generate added heat; such heat may be used to heat up the devicewhen necessary. If a higher temperature differential is required, inaddition to that available from a single TEC, one or more additionalTECs may be interposed in tandem. In some other configurations of thesystem, the thermal head does not include a TEC.

The heat spreader 32 (with its imbedded temperature sensor 39) and theTEC or TECs 33 jointly form the so-called thermal contactor part 35 ofthe thermal head, which has a length ranging from 20-100 mm. In someconfigurations of the head, as illustrated in FIG. 5 the thermalcontactor is detachable and several different interchangeable thermalcontactors may be provided, differing, for example, in the shape of theheat spreader and/or in the type of TEC or TECs, possibly also lacking aTEC altogether. Any of the thermal contactors may be attached to thehead—to be used with corresponding types of electronic devices. As seenin FIG. 5, the thermal contactor has a number of electrical connectors36, configured to engage matching connectors within the body of the headand serving to provide electrical connections to any temperature sensorsand any heating elements within the heat spreader. It is noted that thedetachability of the thermal contactor is also advantageous forfacilitating the replacement of the TEC, which is a component typicallyprone to faults.

The evaporator 40 is the part of the temperature-forcing head that is influid communication with the refrigerant supply- and return tubes 21 and22. It is formed as a closed chamber, with an inlet port and an outletport, to which the ends of supply tube 21 and return tube 22 arerespectively connected. FIG. 4 shows a preferred embodiment ofevaporator 40 in top open view, wherein its interior is seen to bestructured as a heat exchanger 34. The structure forms a maze-likecanal, or passageway, through which the refrigerant flows from the inletport (above point A in the drawing) to the outlet port (above point B).It thus presents a relatively large surface, over which any fluidflowing through the evaporator may readily come in contact and exchangeheat therewith. Various configurations of heat exchanger 34 may employdifferent geometric shapes to achieve such a large surface, including,but not limited to, fins, pin-like or conical protrusions, and aplurality of passageways in parallel.

Preferably, a temperature sensor 37 is imbedded in the body of theevaporator 40 (FIG. 3)—most preferably at its heat exchanger portion 34,as illustrated in FIG. 4. Its output signal is fed, over a pair of wires(not shown), through cable 23, to the control unit 15. It serves toenable controlling the temperature of the heat exchanger (by meansdescribed below) so as to keep it at a level appropriate for dissipatingheat from the DUT at its desired temperature level.

Operation of the system, with a thermal head in the configuration ofFIG. 3, will now be described for each of two operational states—low-and high temperature at the device under test. The operational state isdetermined by the test requirements and is conveyed to the controller 15from the test equipment by a suitable communication path (not shown).Switching between the two states is preferably effected either bychanging the duty cycle of the compressor between a low value (forexample 10% of the time, possibly even 0%, i.e. no operation) and a highvalue (for example 90% of the time, possibly even 100%, i.e. fulloperation) or by changing the operational speed of the compressor and/orby switching the bypass conduit 19 between open and closed states. Itwill be appreciated that any such switching will cause relatively fasttransition between the two temperature states at the device under test.

In the low temperature state, bypass conduit 19 is closed. Action ofcompressor 12 causes pressure to be built up in the refrigerantthroughout the upstream passageway up to the expansion valve 18.Resulting high-pressure and high-temperature gas, flowing throughcondenser 13, is cooled by atmospheric heat exchange (aided by blower14) and is thus converted to liquid (still under high pressure). Thepressure in the liquid refrigerant is reduced once it flows through theexpansion valve, whence it flows, under low pressure, through supplytube 21 into the evaporator 40 (through its inlet port). The interior ofthe latter is kept at a low pressure, due to the drawing action ofcompressor 12 (through return tube 22). The liquid refrigerant flowsthrough the passageway of heat exchanger 34, where it comes in contactwith the large surface of its walls, absorbing the heat that has beendischarged thereto from the heat spreader and consequently evaporatinginto gaseous state at a highly reduced temperature. The resultant gasflows, through the outlet port of the evaporator and return tube 22,back to the inlet of compressor 12, whence it is recycled.

The heat dissipated from heat exchanger 34 lowers its temperature, whichin turn allows it to absorb heat from the adjacent face of TEC 33,lowering its own temperature. Electric current made to flow through theTEC in, say, the forward direction causes its other face to be attemperature that is, by a certain degree, lower than that of the firstface, allowing it to absorb heat from the heat spreader 32, thuslowering also the latter's temperature, which similarly cools the deviceunder test. The temperature of the heat spreader 32 is monitored,through sensor 39, by controller 15, which accordingly adjusts thevoltage applied to TEC 33 so as to keep the sensed temperature at thedesired value. It will be appreciated that the time constant of such acontrol loop is very short, resulting in a very stable temperature atthe device.

For coarser temperature control, as for example in setting the nominalforcing temperature at a level higher than the minimum attainable orwhen the control range must extend beyond what is achievable by the TECalone, the compressor is preferably operated intermittently, that is—italternately operates for a given time period and rests for anotherperiod. The relative length of the operating period is called the dutycycle and is denoted as a percentage. During operation, full cooling iseffected, as described above, whereas during the rest period therefrigerant remains gaseous. The frequency of such cycling is highenough to cause any resulting temperature variations in the heatexchanger to remain below a desired value, aided by its heat latency.The remaining temperature variations may be compensated for by thecontrolled operation of the TEC, as described above. The higher the dutycycle, the greater the average cooling effect and thus the lower thenominal device temperature. An alternative, or additional, coarsetemperature control may be similarly provided by intermittently openingand closing the bypass conduit 19.

Additional control over the heat dissipation process and on theresultant temperatures, may be exerted by adjusting the stricture oropening of the expansion valve 18, thus controlling the rate of flow ofthe refrigerant and its pressure differential; the rate of flow affectsthe rate of heat dissipation within the heat exchanger 34, while thepressure differential influences the lowest temperature achievable bythe evaporation process therein. A similar effect may be provided byvarying the running speed of the compressor. It is noted that all thesemeans provide a relatively fast response time whereby the temperature ofthe DUT is switchable at a rate of 0-150° C./min, and preferably at arate of 50-150° C./min,—again contributing to temperature stability.When a temperature sensor 37 is imbedded in the body of the heatexchanger 34 (or generally in the evaporator), its signal is fed to thecontrol unit 15, where it is used as a feedback signal in controllingthe temperature of the heat exchanger by any of the means recountedabove.

In a high-temperature state in which no heat dissipation from the DUT isdesired, bypass conduit 19 is preferably open. The pressure in therefrigerant is thus not allowed to be lowered to a level at which itcould liquefy and therefore it remains gaseous and, moreover is returnedfrom the expansion valve directly to the compressor throughout the flowcycle. Alternatively the compressor 12 may be shut down altogether.Since now only a negligible cooling effect takes place in theevaporation chamber as a result of the remaining residual refrigerantthat is undergoing evaporation, there is practically no heat dissipatedfrom heat exchanger 34, resulting in a chain of rising temperatures,through the TEC and the heat spreader to the device under test. Thelatter's temperature is thus allowed to rise, by the effect of heatgenerated within it by its own operation during testing. If this is notsufficient, a voltage applied across TEC 33 in the reverse directioncauses the temperature of its lower face to rise, which further warmsthe heat spreader and thence—the device. The heating effect in the TECmay be due to both the Peltier effect and ohmic losses. For extremecases, a resistive electric heater (not shown) may be placed in thethermal head (e.g. within the heat spreader) and a current may becontrollably driven therethrough. On the other hand, for the case thatthe device generates heat at a rate greater than that dissipated by itsenvironment, some heat dissipation by the thermal head would be calledfor and then active cooling may be applied as described above for thelow temperature case—albeit at a suitably low cooling rate.

The temperature level at the device is, again, accurately maintained bycontrolling the magnitude of the voltage applied to TEC 33 or to theresistive heater through a closed loop, involving sensor 39 andcontroller 15.

In certain configurations of thermal head 30, intended for testingdevices where the lowest required forcing temperature is well above thatachievable by the system in full operation, the head does not include aTEC, but preferably includes, instead, a simple electrically resistive(ohmic) layer between the heat exchanger 34 and the heat spreader 32.Alternatively a heating element may be imbedded in the heat spreader.Electrical current is controllably driven through the resistive layer(or the heating element) so as to provide additional heat that must bedissipated by the cooling system, thus, in effect, raising the forcingtemperature of the device by a given amount. This resistive arrangementserves for finely and accurately controlling the device temperature, ina closed-loop manner similar to that effected by the TEC in thepreviously described configuration.

FIG. 6 illustrates another embodiment of the invention wherein anextreme low device temperature is achievable by employing a cascading,two stage refrigeration cycle.

In the schematic illustration of the refrigeration cycle, the firststage refrigerant flows in closed loop conduit circuit 62 and secondstage refrigerant, which may be of a different type than the first stagerefrigerant to provide an increased cooling rate, flows in closed loopconduit circuit 72. The temperature of the first stage refrigerant mayrange from 0 to −60° C. and the temperature of the second stagerefrigerant may range from 0 to −70° C., while their pressure may rangefrom 0.7 to 24 bar. The higher pressure levels are sufficient tomaintain the refrigerant in a liquid phase.

The structure of the central unit is similar to that of the single stagecycle, although provided with an additional compressor, conduit circuitand heat exchanger, and therefore need not be described, for brevity.

The first stage refrigerant is pressurized by first compressor 64 to ahigh pressure P1 and a high temperature T1, and is then cooled bycondenser 65, e.g. an air-cooled type, to a temperature T2. After flowcontrol device 66, e.g. a capillary tube, lowers the pressure of thefirst stage refrigerant to P3 and its temperature to T3, generally below0° C., the first stage refrigerant is delivered to heat exchanger 71, inorder to cool the second stage refrigerant exiting second compressor 74.The first stage refrigerant exiting heat exchanger 71 at a highertemperature of T4 and a higher pressure of P4 is delivered to firstcompressor 64.

The second stage refrigerant is pressurized by second compressor 74 to ahigh pressure P5 and a high temperature T5 greater than P1 and T1,respectively, and is then delivered to heat exchanger 71, whereat it iscooled by the first stage refrigerant to pressure P6 and temperature T6greater than P3 and T3, respectively. The cooled second stagerefrigerant flows to evaporator 77 retained in the temperature-forcinghead, e.g. a labyrinth type evaporator, and is evaporated as a result ofheat transfer from the DUT, producing a pressure P7 and a temperature T7less than P3 and T3, respectively. The second stage refrigerant exitingevaporator 77 is delivered to second compressor 74.

EXAMPLE 1

A field-programmable gate array (FPGA) device was subjected totemperature forcing at extreme temperatures ranging at an extreme hightemperature between 135 and 200° C. and at an extreme low temperaturebetween 0 and −60° C. Freon R404A was used as the refrigerant.

The compressor pressurized the refrigerant to a pressure of 250-300 psi,resulting in a temperature of 50° C., and provided a suction pressure of10-20 psi. The refrigerant was cooled by an air-cooled type condenser toa temperature of 30° C. A capillary tube lowered the pressure of therefrigerant to 20 psi and its temperature to −30° C.

The cooled refrigerant was delivered to a labyrinth type evaporatorretained in the temperature-forcing head, and was evaporated as a resultof heat transfer from the FPGA device, producing a temperature of −55°C. at a heat dissipation rate of up to 1 kW.

The compressor operated continuously during the cooling phase, and wasnot operated during the heating phase. A thermoelectric cooler providedin the temperature-forcing head was alternately operated and deactivatedfor a frequency ranging from 20-1000 Hz during both the cooling andheating phases.

The FPGA device was maintained at a constant temperature that did notfluctuate more than a temperature difference of 0.1° C. between tworegions thereof. During testing, the temperature of the FPGA device wasswitched from an extreme high temperature 200° C. to an extreme lowtemperature of −60° C. a rate of 50-70° C./min, within a time period of3.7-5.2 min.

EXAMPLE 2

A FPGA device was subjected to temperature forcing at extremetemperatures ranging at an extreme high temperature between 135 and 200°C. and at an extreme low temperature between −30 and −70° C. Freon R23was used as the refrigerant in thermal contact with the device. Thetemperature-forcing head was not provided with a thermoelectric cooler,but rather the low temperatures were made possible by a two stagerefrigeration cycle and the high temperatures were achieved by the useof a resistive heater.

In the first stage, a first compressor pressurized the R404A refrigerantto a pressure of 250-300 psi, resulting in a temperature of 50° C., andprovided a suction pressure of 10-20 psi. The refrigerant was cooled byan air-cooled type condenser to a temperature of 30° C. A capillary tubelowered the pressure of the refrigerant to 20 psi and its temperature to−30° C.

In the second stage, a second compressor pressurized the R23 refrigerantto a pressure of 400-600 psi, resulting in a temperature of 70-80° C.,and provided a suction pressure of 10 psi. The refrigerant exiting thesecond compressor was delivered to a plate type heat exchanger, and wasthereby cooled by the R404A refrigerant circulating in separatealternating plate-shaped chambers to a temperature of −20° C. The cooledR23 refrigerant was delivered to a labyrinth type evaporator retained inthe temperature-forcing head, and was evaporated as a result of heattransfer from the FPGA device, producing a temperature of −70 to −80° C.at a heat dissipation rate of up to 1 kW.

The first and second compressors operated continuously during thecooling phase. During the heating phase, a single resistive flat heaterproviding a heat influx of 0-1 kW was used. When the heater was operatedfor a duration ranging from one msec to one sec, a heating pulse of 1 Wwas generated.

The FPGA device was maintained at a constant temperature that did notfluctuate more than a temperature difference of 0.1° C. between tworegions thereof. During testing, the temperature of the FPGA device wasswitched from an extreme high temperature 200° C. to an extreme lowtemperature of −70° C. a rate of 150° C./min, within a time period of1.8 min.

While some embodiments of the invention have been described by way ofillustration, it will be apparent that the invention can be carried outwith many modifications, variations and adaptations, and with the use ofnumerous equivalents or alternative solutions that are within the scopeof persons skilled in the art, without exceeding the scope of theclaims.

1. A temperature-forcing system, for controlling the temperature of anelectronic device under test, comprising: a) a temperature-forcing head,including a face positionable in thermal contact with said device, andan evaporator, in direct or indirect thermal contact with said face; andb) a refrigerant circulation subsystem, including a compressor, acondenser, a flow control device for inducing a pressure drop in saidrefrigerant, and a conduit circuit through which said refrigerant isflowable, wherein said subsystem is configured to cooperate with saidevaporator so as to define at least one closed loop through which acorresponding bi-phase refrigerant is circulatable, so that, duringcirculation, said refrigerant is maintained in a liquid phase betweenthe compressor and the flow control device and in a gaseous phase whileflowing through the evaporator, wherein the temperature of said deviceis switchable by means of said head at a rate of 50 to 150 degreesCelsius per minute.
 2. The system of claim 1, wherein the temperature ofthe electronic device is maintained at a substantially constanttemperature by means of the temperature-forcing head.
 3. The system ofclaim 2, wherein a maximum difference between an instantaneous highdevice temperature and an instantaneous low device temperature is nomore than 0.10° C.
 4. The system of claim 1, wherein the rate of heatdissipation from the electronic device is controllable by temporarilydeactivating the compressor or by adjusting the flow rate of therefrigerant through the evaporator.
 5. The system of claims 1, whereinthe evaporator is configured as a heat exchanger.
 6. The system of claim1, wherein said head further includes a thermo-electric cooler, one ormore resistive heaters, or more temperature sensors.
 7. The system ofclaim 1, wherein said head is formed as two parts that are mutuallyattachable and detachable, one part being a thermal contactor, whichincludes said face configured to be put in thermal contact with a deviceunder test.
 8. The system of claim 7, wherein said thermal contactorfurther includes a thermo-electric cooler.
 9. The system of claim 1,wherein said subsystem further includes a bypass conduit, configured toselectively provide a flow path for the refrigerant that avoidscirculation through the evaporator.
 10. The system of claim 1, whereinsaid subsystem is operative to circulate the refrigerant intermittently,with a variable duty cycle, or at a variable rate of flow.
 11. Thesystem of claim 1, wherein the flow control device is an adjustableexpansion valve.
 12. The system of claim 1, wherein the conduit circuitcomprises a tube assembly, attached to said head and configured to passsaid refrigerant from said subsystem to the evaporator and back from theevaporator to said subsystem.
 13. The system of claim 12, wherein thetube assembly is connectable to, and detachable from, said subsystem.14. The system of claim 1, wherein the circulation subsystem is a twostage refrigeration cycle by which a first stage refrigerant pressurizedby a first compressor is delivered to a heat exchanger whereat it coolsa second stage refrigerant pressurized by a second compressor to apressure greater than the pressure generated by said first compressor,for an increased cooling effect.
 15. The system of claim 1, comprisingone or more additional temperature-forcing heads, each similar to saidtemperature-forcing head, wherein said subsystem is configured tosimilarly circulate said bi-phase refrigerant also through theevaporator of each of said additional heads.
 16. The system of claim 1,wherein the refrigerant is selected from the group consisting of Freon,Freon R22, Freon R23, Freon R134, Freon R134A, Freon R404A, Freon R408A,Freon R507, Freon R717, Freon R134, carbon dioxide, and nitrogen.
 17. Atemperature-forcing head, for controlling the temperature of anelectronic device under test, comprising a thermal contactor, configuredwith a face adapted for thermal contact with the device, and anevaporator, in direct or indirect thermal contact with the thermalcontactor and configured to be connectable to a circulation system forcirculating a bi-phase refrigerant in a gaseous phase through theevaporator and in a liquid phase through a condenser, wherein thetemperature of said device is switchable by means of said head at a rateof 50 to 150 degrees Celsius per minute.
 18. The thermal head of claim17, wherein the evaporator is formed as a heat exchanger or the thermalcontactor includes a thermo-electric cooler.
 19. A method fordissipating heat from a thermal head while in thermal contact with anelectronic device under test, comprising: (i) providing a bi-phaserefrigerant and a circulation system therefor, in fluid communicationwith a portion of the thermal head; and (ii) causing at least some ofsaid refrigerant, while in liquid phase, to come in thermal contact withsaid portion, whereby at least some of the refrigerant evaporates,wherein the temperature of said device is switchable by means of saidhead at a rate of 50 to 150 degrees Celsius per minute.
 20. A method forforcing the temperature of an electronic device under test, comprising:(i) providing a thermal head, including an evaporator part, and placingit in thermal contact with the device; (ii) connecting the head to acirculation system and causing said system to circulate a bi-phaserefrigerant through the evaporator part; and (iii) when heat dissipationfrom the device is required, causing at least part of said refrigerantto enter the evaporator in liquid phase, whereby it dissipates heattherefrom by evaporation, wherein the temperature of said device isswitchable by means of said head at a rate of 50 to 150 degrees Celsiusper minute.